The present invention relates to fuser apparatus for electrostatographic printing machines and in particular to a roll fuser release agent management (RAM) system.
In the process of xerography, a light image of an original to be reproduced is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto in one of various ways, for example, as by heat and pressure.
In order to affix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky while simultaneously applying pressure. This action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
One approach to heat and pressure fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to effect heating of the toner images within the nip. By controlling the heat transferred to the toner, virtually no offset of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the "hot offset" temperature of the toner whereat the toner particles in the image areas of the toner liquefy and cause a splitting action in the molten toner resulting in "hot offset." Splitting occurs when the cohesive forces holding the viscous toner mass together is less than the adhesive forces tending to offset it to a contacting surface such as a fuser roll.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e. "cold" offsetting); by imperfections in the properties of the surface of the roll; or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them there. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the backup roll during periods of time when no copy paper is in the nip.
Moreover, toner particles can be picked up by the fuser and/or backup roll during fusing of duplex copies or simply from the surroundings of the reproducing apparatus.
One arrangement for minimizing the foregoing problems, particularly that which is commonly referred to as "offsetting," has been to provide a fuser roll with an outer surface or covering of polytetrafluoroethylene, known by the tradename Teflon to which a release agent such as silicone oil is applied, the thickness of the Teflon being on the order of several mils and the thickness of the oil being less than 1 micron. Silicone based (polydimethylsiloxane) oils which possesses a relatively low surface energy, have been found to be materials that are suitable for use in the heated fuser roll environment where Teflon constitutes the outer surface of the fuser roll. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
A fuser roll construction of the type described above is fabricated by applying in any suitable manner a solid layer of adhesive material to a rigid core or substrate such as the solid Teflon outer surface or covering of the aforementioned arrangement.
Various systems have been used to deliver release agent fluid to the fuser roll including ones that use oil soaked rolls and wicks with and without supply sumps as well as oil impregnated webs. The oil soaked rolls and wicks generally suffer from the difficulty in that they require a sump of oil to replenish the roll and the wick as its supply of release agent is depleted by transfer to the fuser roll. Furthermore, a wick suffers from the difficulty of a relatively short life, around 10,000 prints. Furthermore, these systems suffer from the further difficulty, in that, their surfaces in contact with the fuser roll are constant whereby contamination particularly by toner and paper can readily occur further reducing valuable life. The web systems, on the other hand are limited in the quantity of oil they can deliver.
U.S. Pat. No. 3,941,558 to Takiguchi discloses a rolled web impregnated with silicone oil for preventing offset. The web has a thickness of 2 mm, a total length of 50 cm, and travels one cm per thousand copies between the supply and take-up rollers. This system transfers about 0.003 cc of oil to the fuser per copy.
U.S. Pat. No. 4,393,804 to Nygard et al. discloses a rolled web system that moves between a supply core and take-up roller. A felt applicator supplies oil from a supply reservoir to the web. The take-up core is driven by a slip clutch at a speed greater than the speed of the pressure roller, thus exerting tension on the web. The web is between one and two mm in thickness and moves at a constant speed of five cm per 200 to 1,000 copies.
U.S. Pat. No. 5,049,944 granted to DeBolt et al on Sep. 17, 1991 relates to apparatus for applying offset preventing liquid to a fuser roll including: a supply core; a rotatable take up core; an oil impregnated web member adapted to be moved from the supply core to the take-up core; a motor mechanically coupled to the take up roll for driving the web member from the supply core to the take up core; a pressure roll in engagement with the web member and positioned to provide a contact nip for the web member with the fuser roll opposite the pressure roll wherein the contact of the web member with the fuser roll transfers oil from the web member to the fuser roll and control means to vary the duty cycle operation of the motor to drive the web member at a relatively constant linear speed at the contact nip. In addition to the oil impregnated web, a foam pinch roll is also impregnated with release agent material to insure that any sections of the web that may have been loaded with an inadequate quantity of silicone oil are supplied with additional release agent material.
In addition, there are several automatic printing machines commercially available. For example, the Canon 3225, 3725, 3000 series, 4000 series and 5000 series products all have liquid release agent impregnated webs supported between a supply roll and a take-up roll and urged into contact with the fuser roll by an open celled foam pinch roll. Additionally, the Xerox.TM. 5028.TM. machine utilizes an oil impregnated web for application of between 0.1 to 0.5 micro liters per copy of release agent material to a heated fuser roll.
Although RAM systems utilizing oil impregnated webs provide an adequate quantity of release agent material for low copy volumes they are not suitable for high volume machines such as printers or for use in machines that produce color copies or prints. Also, roll fusers used for fixing color images require a greater quantity of oil application than oil impregnated webs are capable of delivering. The release properties of fuser rolls operated with insufficient release material degrade to an unacceptable level.